An acoustic resonator typically comprises a layer of piezoelectric material sandwiched between two plate electrodes in a structure referred to as an acoustic stack. Where an input electrical signal is applied between the electrodes, the reciprocal or inverse piezoelectric effect causes the acoustic stack to mechanically expand or contract depending on the polarization of the piezoelectric material. As the input electrical signal varies over time, expansion and contraction of the acoustic stack produces acoustic waves that propagate through the acoustic resonator in various directions and are converted into an output electrical signal by the piezoelectric effect. Some of the acoustic waves achieve resonance across the acoustic stack, with the resonant frequency being determined by factors such as the materials, dimensions, and operating conditions of the acoustic stack. These and other mechanical characteristics of the acoustic resonator determine its frequency response.
Many acoustic resonators have a frequency response with a passband characterized by center and cutoff frequencies. Such a frequency response makes these acoustic resonators attractive for a variety of electronic applications, with one example being frequency filters for communication equipment. Unfortunately, however, the passband of an acoustic resonator may vary with changes in temperature. For instance, as the ambient temperature increases, the passband may move toward lower frequencies because added heat tends to soften the materials that typically form the acoustic resonator, reducing their acoustic velocity.
To address this temperature dependent behavior, acoustic resonators are commonly designed with a relatively wide passband to allow for changes in the ambient temperature. Such a wide passband typically requires the acoustic resonator to have a relatively high electromechanical coupling coefficient (Kt2), which may be difficult to achieve and may require additional processing steps such as scandium doping of an aluminum nitride (AlN) piezoelectric material. Moreover in some filters, such as those designed to operate in Band 13, the passband is not allowed to move because it may encroach on other (e.g., safety) bands.
In an effort to provide a stable passband in the presence of temperature changes, some acoustic resonators incorporate a temperature compensating material to counteract temperature-induced changes in the acoustic velocity of other resonator materials. For instance, an acoustic resonator may include an embedded layer of temperature compensating material whose acoustic velocity increases with increased temperature in order to counteract a reduction in the acoustic velocity of the piezoelectric material and electrodes.
One drawback of using the temperature compensating material is that it tends to redistribute acoustic energy within the acoustic stack, which may also increase excitation of spurious modes and diminish various performance metrics such as series resistance (Rs), parallel resistance (Rp) and overall quality factor (Q) across the pass-band spectrum. Accordingly, certain structures can be built into the acoustic resonator to counteract the reduction of these and other metrics. For example, air-bridges can be built over peripheral portions of the top electrode to reduce acoustic losses produced by interactions with an underlying substrate, and add-on frames can be formed over the top or bottom electrodes to minimize scattering of acoustic waves at the top electrode edges. These structures, in combination with the use of the temperature compensation material, tend to complicate the manufacture of the acoustic resonator. Accordingly, there is a general need for improved techniques for providing temperature compensation in acoustic resonators used in filters and other applications.